FR2574397A1 - PROCESS FOR PREPARING HEXAGONAL MAGNETIC FERRITES - USE OF SUCH FERRITES AS BASIC MATERIALS FOR MAGNETS OR FOR MAGNETIC RECORDING - Google Patents

Abstract

PROCESS FOR PREPARING MAGNETIC HEXAGONAL FERRITES AND ESPECIALLY BARIUM HEXAFERRITE BY HYDROTHERMAL SYNTHESIS REACTION BETWEEN AN ALKALINE FERRITE AND A COMPOUND RELEASING AN ALKALINE EARTH ION IN WATER. MAGNETIC HEXAGONAL FERRITES CAN BE USED AS A MATERIAL FOR PERMANENT MAGNETS AND FOR HYPERFREQUENCY COMPONENTS OR FOR MAGNETIC RECORDING.

Description

PROCESS FOR THE PREPARATION OF AIMANIA HAGOLE FERRITES

  USE OF THESE FERRITIES CCOMME BASIC MATERIALS

  The present invention relates to a process for preparing in a hydrothermal medium fine particles of hexagonal ferrites useful in particular as base materials for permanent magnets or for microwave components as well as for magnetic recording. The invention also relates to hexagonal ferrites obtained by their manufacturing process and the use of these

  ferrites in magnetic applications.

  Processes for obtaining magnetic hexagonal ferries, and in particular hexaferrite barium, are already known. The preparation of l. Barium hexaferrite has been described in numerous patents and articles and in particular in the article by H. HIBST published in Angew. Chem.

Int. Ed. 21 [1982] 270-282.

  Among the processes that can be used, mention will be made of the ceramic process, the coprecipitation and annealing process, the melting process for mixtures of oxides or molten salts or hydrothermal synthesis. The hydrothermal synthesis technique consists in reacting the constituents by solubilization-coprecipitation in an aqueous medium under pressure and at a high temperature [150 ° C. to 400 ° C.]. This reaction was first applied to the synthesis of barium hexaferrite by the reaction of iron oxide and barite: thus a dispersion of iron oxide and an aqueous solution of barite were reacted. Although it leads to the formation of hexaferrite barium, the results are not satisfactory [yield, purity] especially because of the insolubility of iron oxides in water. This is why it has been recommended to increase the solubility of the reagents by the choice of more appropriate precursors and / or by the use of a very basic medium [introduction of alkaline bases]. The addition of alkaline base also makes it possible to increase the nucl. by reducing the particle size

  manufactured and lowering the reaction temperature.

  Precursors to. iron bases intended to remedy the difficulty of dissolving iron oxides in water, and which have been recommended in the context of hydrothermal synthesis have been ferric or ferrous salts such as chlorides or nitrates all soluble

  in the water, goethite has freshly prepared.

  Thus, according to the European Patent Application 102 819 of TODA, barium hexaferrites obtained in hydrothermal synthesis in a very basic medium are described from goethite a; the platelet-shaped magnetic particles are usable for magnetic recording; they have a low surface area [less than 5m / g], a magnetization of about 35 to 60 uem / g and a

  coercive field less than 1,000 oersteds.

  However, all processes for the preparation of barium hexaferrite by hydrothermal synthesis as described in the prior art have a number of defects which are harmful to a transposition.

  satisfactory on the industrial level.

  The use of soluble iron salts (chloride, nitrate ...) involves very significant corrosion phenomena prohibiting industrially the use of stainless steel and requiring the use of autoclaves particularly resistant coatings [autoclaves

  must for example be lined in gold].

  Furthermore, the purity of the hexagonal ferrites is often reduced by the presence of other non-ferromagnetic phases which must be separated, which has the effect of limiting the magnetization and the field

coercive.

  On the other hand productivity is low and yields are

far from being quantitative.

  There was therefore the problem of developing a hydrothermal synthesis process of haxagonal ferrites making it possible to obtain, with high productivity and a very high yield, ferromagnetic fine particles of hexagonal hexaferrites having good magnetic properties, the process being carried out in relatively moderate conditions of pressure and temperature and in a low corrosive environment that does not necessarily require a specially coated autoclave

resistant.

  The invention responds precisely to this purpose.

  It has been found and this is the object of the present invention a method of prepairing ferromagnetic hexagonal ferrites of metal. alkaline earth by hydrothermal reaction between a compound releasing into water an alkaline earth ion and an iron precursor, the process being characterized in that

  uses as precursor an alkaline ferrite.

  The alkali ferrites which can be used in the context of the present invention may be compounds which differ in the nature of the alkaline cation and / or in the nature of the crystalline phase which they constitute. By way of illustration, mention may be made of iron ferrites of formula FeNaO 2, varieties a and B, potassium ferrite. Alkaline ferrites are compounds of their own known but never used as a precursor in hydrothermal synthesis. The alkaline ferrites are easily obtained for example by heating in air, at high temperature, a mineral mixture! consisting of an iron oxide of

  low or coarse particle size and an alkaline carbonate.

  Advantageously, as part of the process, sodium ferrite variety B is used as the alkali ferrite. This is easily obtained by heating at around 1000 ° C. for several hours at a temperature of

  mixture of carbonate of soda and iron oxide [Fe203 or Fe304].

  The compound which liberates an alkaline earth ion in water is not of itself critical: it may be an alkaline earth hydroxide, an organic or inorganic salt or a complex compound of alkaline earth metal. Among the compounds releasing an alkaline-earth ion in water, mention may be made, for example, of hydroxides,

  chlorides, acetates and carbonates.

  Preferably, in the context of the present invention, the compounds of barium, strontium, or calcium, and especially the compounds of barium, will be used as the alkaline earth metal component. According to an advantageous variant

  the barium compound is a hydrated barium Ba (OH) 2, 8 H 2 O.

  According to a variant of the present invention it is also possible to use a mixture of alkaline ferrite and / or a mixture of compound releasing in water at least 2 alkaline earth ions of different nature. Of course the use of a mixture of alkaline ferrite and a mixture of compounds releasing into water at least two alkaline earth ions must be considered as a variant of the present invention. The hydrothermal synthesis reaction is carried out at a temperature above 100 C and below the critical point of the aqueous mixture; the temperature is usually between 2000 and 400 C; as will be explained below, the choice of temperature has a great influence on the size, crystallinity and magnetic properties of hexaferrite; the pressure is variable according to the operating conditions and in particular according to the degree of filling of the autoclave. The pressure is generally less than 500 bars and

  usually between 30 and 300 bars.

  The relative proportion of the various reagents can be very variable. As a general rule, an excess of the liberating compound in water is added to the alkaline earth ion. In the case of barium the atomic ratio barium / iron is usually between 1/6 and 1/12 (1/12 ratio

  stoichiometric) and is preferably between 1/8 and 1/10.

  The hydrothermal process is generally carried out without

  initially add alkaline base with the two main reagents.

  According to a variant of the present invention it is possible to initially add an alkaline base with the two main reagents. the amount of alkaline base added depends on the quality of the water used to control the pH and is then such that there is initially for one mole of alkaline ferrite from 0.5 to 20 moles of alkaline base the amount of added water being less than or equal to cc; advantageously the added base is sodium hydroxide. This addition of alkaline base thus makes it possible by choosing the value of the temperature to increase the nucleation phenomena and by the

reduce the size of the grains.

  The value of the filling factor of the autoclave can be

  very variable; advantageously the filling coefficient will be very

  high to increase productivity; the amount of water used can be relatively small: the mixture of alkaline ferrite powder + compound releasing into water the alkaline-earth ion can

  100% to 300% of the weight of the water involved.

  It is therefore observed that, unlike the previous processes, the quantity of product formed is very large for a given volume of autoclave, since the quantity of water used can be very high.

low.

  The hydrothermal synthesis process is carried out for varying periods of time, obviously depending on the temperature, the pressure, the nature of the reagents and their proportion. Usually

  durations are between 1 hour and several hours.

  When the reaction is complete or it is decided to stop it, the reaction medium is brought back to ordinary temperature, preferably

  by rapidly cooling the autoclave.

  The hexagonal hexaferrite produced is purified by conventional methods by removing reagent residues as well as non-ferromagnetic formed phases; it is possible in particular to implement a separation treatment based on the differences of the

magnetic properties.

  According to a preferred treatment of the present invention, the hexaferrites are washed with aqueous solutions of dilute acid such as hydrochloric acid, nitric acid, which generally allow the non-ferromagnetic phases of the present system to be well removed; acid dilution is not critical: it is usually

  between 5 and 50%. A dilution of 20% is fine.

  According to another preferred treatment of the present invention, it is also possible to anneal the hexaferrite produced by the hydrothermal synthesis. This annealing is usually carried out under air for several hours at a temperature between 700 and 1200 C; this temperature is advantageously located around

] .000 C.

  Finally according to another treatment of purification of hexaferrites

  it is possible to combine in a variable order an acidic wash and an annealing.

  It is this well-chosen combination of treatments that makes it possible

  genera] to obtain optimum magnetic properties.

  The process according to the invention makes it possible, as has already been said, to manufacture hexagonal hexaferrites that can be used either in permanent magnets or for microwave components or else as

  magnetic recording media.

  It is known that the requirements in terms of magnetic properties are different depending on whether they are particles for magnets

  permanent or for recording media.

  The method according to the invention makes it possible to obtain ferromagnetic particles for permanent magnets; these are characterized by a high value of magnetization and a coercive field of at least 1,200 oersteds. In order to obtain such particles, the hydrothermal synthesis reaction is carried out at a relatively moderate temperature [225 to 300 ° C]; the ferrites produced then preferably undergo an annealing treatment which generally makes it possible to increase the value of the magnetization and the value of the coercivity. The particles produced have a specific magnetization of at least 65 uem / g

  ] And a coercive field of at least 2500 and possibly exceeding 2500 oersteds.

  Of course the precision that has just been given can not in any way

  to be considered a limitation.

  The method according to the invention also makes it possible to obtain hexagonal hexaferrite particles that can be used as magnetic recording media. To achieve this, the hydrothermal synthesis reaction is carried out at a relatively high temperature of the order of 300 C or more. After a purification treatment the particles

  have a specific magnetization of at least 65 uem / g and a value.

  coercive field less than 1200 oersteds.

  Another object of the present invention is constituted by the hexagonal hexaferrites obtained by the method as above.

  described with all its variants.

  Finally, a final object of the present invention is constituted by the use of hexagonal ferrites manufactured according to the process of the present invention, as magnetic materials for magnets.

  permanent or for magnetic recording.

  The following examples illustrate the invention.

Example 1

  39.40 g of baryta octahydrate Ba (OH) 2, 8 HE 2 O are added to the autoclave, as is 11060 g of sodium B ferrite NaFeO 2 and cm 2 of water, the reaction mixture in which the barium atom / iron is equal to 1/8 is heated for 4 hours at 395 ° C, the pressure reaches 200 bar, quenched cooling, the reaction mass which is in particulate form is first washed with a 20% aqueous solution of nitric acid, then with water

  pure and finally annealed at 1000 C in air for 2 hours.

  X-ray diffraction demonstrates the presence of a well-crystallized M phase characteristic of BaFe209 barium hexaferrite. Hexaferrite was obtained with a yield of 90% and

a purity of 95%.

  The marnetic properties are as follows: - specific magnetization 66.2 uem / g (66.6 Am2 / hg) - coercive field 1250 oersted The measures of magnetization were made while the maximum field was 2.66 tesla

  The specific surface of the grains is 2.7 m 2 / g.

Example 2

  The hydrothermal synthesis was carried out according to the method described in Example 1 but at a temperature of 250 ° C. for 4 hours 30 minutes.

  [maximum pressure reached: 40 bars].

  After quenching and purification according to Example 1, washing and then annealing, crystals of hexaferrite of barium whose field is obtained are obtained.

coercive is 3600 oersteds.

  The magnetization is 54.5 uem / g The specific surface is 5.3 cm2 / g If we proceed to ar-ord an annealing followed by a wash, we have the properties: specific magnetization 65 uem / g coercive field 2660 oersteds - specific area 2.5 cm2 / g

Example 3

  The hydrothermal synthesis is carried out according to the method described in Example 1 but at a temperature of 345 C for 6 hours [maximum pressure reached: 130 bar], while the volume of water initially

introduced is 60 cm.

  After quenching and purification according to Example 1, barium hexaferrite crystals were obtained having the following magnetic properties: specific magnetization: 64.2 μm / g - coercive force: 1460 oersted specific surface area: 3.7 m 2 / g

Example 4

  According to the procedure of Example 1, the hydrothermal synthesis reaction is carried out at 400 ° C. for 6 hours, using

  initially 90 cm3 of water and 150 g of barium-ferrite powder mixture.

  In experiment 4a the barium / iron atomic ratio was

  1/6; in experiment 4b the atomic ratio was 1/10.

  After quenching and purification according to Example 1,

  barium hexaferrite crystals having the following properties:

example 4 a

  - specific magnetization: 67.7 uem / g - coercive field: 1250 oersteds

example 4b

  - specific magnetization: 66.4 uem / g - coercive field: 1350 oersteds

Claims (4)

  1 - Process for the preparation of ferromagnetic hexagonal ferrites of alkaline earth metal by hydrothermal reaction between a compound releasing into water the alkaline earth ion and an iron precursor, the process being characterized in that   uses as precursor an alkaline ferrite.   2 - Process according to claim 1 characterized in that the alkali ferrite is sodium ferrite variety B. 3 - Method according to one of claims 1 and 2 characterized in that the compound releasing in water the alkaline ion earthy is a   hydroxide, or an organic or mineral salt.   4 - Process according to one of claims 1 and 2 characterized in that the compound releasing into water the alkaline earth ion is a composed of barium or strontium.   - Method according to one of claims 1 and 2 characterized in that the compound releasing into water the alkaline earth ion is baryta. 6 Process for preparing hexagonal ferrites according to one   of claims 1 or 2 characterized in that after reaction the   ferrites are purified by acid wash or acid wash and annealed.   7 - hexagonal ferrites obtained according to one of the claims   preceding. 8 - Use of the hexagonal ferrites obtained according to one   Claims 1 to 6 as Magnetic Magnet Materials   permanent, for microwave components or for magnetic recording.